You can test how big you want it on a pencil or pencils. Even if you think an 8 by 3 is too big do it anyway because it looks bigger than it is going to look when you are finished.
You set a Lego outline for your pencil holder.
Do one or two layers on top of that but make sure they are not exactly the same pieces or it would just fall apart.
Now look at the bottom of the thing and push in a little and it is wobbling add 1or 2 layers. It does not matter if they go off the edge.
Then keep on adding to the top. Then you get your Lego pencil holder.